This invention relates to a method of drying quaternary ammonium compounds for use as accelerators in the polymerization of 2-pyrrolidone.
Quaternary ammonium compounds are notoriously hygroscopic and difficult not only to dry but to keep dry. Even momentary contact with air at normal humidities will cause these substances to pick up enough water so that they cannot be used as additives in the polymerization of 2-pyrrolidone which requires substantially anhydrous conditions.
The anionic polymerization of 2-pyrrolidone is well known in the art and was first disclosed in U.S. Pat. No. 2,638,463. Improved methods of polymerizing 2-pyrrolidone are disclosed in many subsequent patents, for example U.S. Pat. No. 3,721,652, but all involve the use of a metallic derivative of 2-pyrrolidone, or a similar derivative, as the catalyst. Preferred metals are the alkali metals except lithium. To initiate the polymerization an additional substance, customarily called an "activator" is required. Preferred activators are C0.sub.2 or SO.sub.2 although many other substances such as N-acetyl pyrrolidone may be used. In order for the polymerization to occur the system must be substantially anhydrous. It has been determined that the presence of more than about 0.1% water by weight based on the 2-pyrrolidone monomer effectively retards or prevents polymerization and less than this amount of water is preferred.
It has been found that the addition of quaternary ammonium salts to the polymerization mixture causes a marked increase in the rate of polymerization provided that the overall moisture content is kept below the 0.1% critical value. Such an increase in the rate of polymer formation is of great commercial importance since prior art methods have provided only comparatively low conversions in 24 hours or longer.
Quaternary ammonium compounds are notoriously very hygroscopic and are therefore nearly always wet. They are also difficult to dry because they are thermally unstable. Drying is usually attempted by heating under a good vacuum and, depending upon the thermal stability of the particular quaternary ammonium compound, much of the water may be removed but usually not all. The operation is at best a compromise between a need to raise the temperature to a point where all the water may be removed and a need to keep the temperature below the point where some thermal decomposition occurs. When intended for addition to a 2-pyrrolidone polymerization mixture, slight thermal decomposition is almost as detrimental as having some water present since the amines formed in the decomposition are inhibitors to the polymerization.
It is therefore the object of this invention to provide a simple and practical method of forming an anhydrous solution or suspension of quaternary ammonium compounds in 2-pyrrolidone free of inhibiting thermal decomposition products.